This invention relates to a superconducting energy storage apparatus connected through an AC-DC converter unit to an electric power system so as to regulate transmission of electric power through the electric power system, and more particularly to the timing control of turning on a persistent current switch used for establishing a persistent current mode while energy is being charged into or discharged from the superconducting energy storage apparatus connected to the electric power system.
With the recent remarkable development of superconducting technology, superconducting coils are now finding more and more useful applications to the fields of nuclear fusion, charged particle acceleration, magnetic floating, medical care, etc. Also, application of a superconducting energy storage apparatus to the field of electric power is now planned so that the apparatus can be used to carry out various functions including stabilization of the electric power system, compensation of fluctuating loads and storage of surplus electric power.
If some kind of serious trouble might occur in the electric power system while the superconducting energy storage apparatus is carrying out the function of stabilization of the electric power system or the function of compensation of a fluctuating load, it is necessary to immediately disconnect the superconducting energy storage apparatus from the electric power system and to shift the operation mode to a persistent current mode so as to prevent further spread of the trouble occurring on the side of the electric power system. Therefore, a persistent current switch used to establish the persistent current mode is required to be turned on at the highest possible speed.
FIG. 5 shows the structure of one form of a prior art superconducting energy storage apparatus including a mechanical type persistent current switch. Referring to FIG. 5, a superconducting coil 3 installed in a cryostat 4 is connected to an AC-DC converter unit 6 through a quench protection unit 5, and the mechanical type persistent current switch 1 is connected in parallel to the AC-DC converter unit 6. The AC-DC converter unit 6 is connected at its power receiving side to an electric power system 9. A control unit 10 is connected to the mechanical type persistent current switch 1 and the AC-DC converter unit 6.
In the prior art superconducting energy storage apparatus having the structure shown in FIG. 5, the persistent current mode is established in a manner as described now. In the normal operation mode, the energy is charged into or discharged from the superconducting coil 3 through the AC-DC converter unit 6 connected to the electric power system 9. The normal operation mode is shifted to the persistent current mode by first operating the AC-DC converter unit 6 as an inverter, and the mechanical type persistent current switch 1 is then turned on in response to an 0N instruction signal applied from the control unit 10, with the result that the direct current, having flowed through the AC-DC converter unit 6, is now shifted to flow through the mechanical type persistent current switch 1, thereby forming a closed loop including the mechanical type persistent current switch 1 and the superconducting coil 3.
Publications relating to such a persistent current switch include JP-A-Hei-1-177839 and JP-A-Hei-4-211105.
In the prior art superconducting energy storage apparatus described above, two steps are required until the mechanical type persistent current switch 1 is turned on. That is, in the first step, the AC-DC converter unit 6 is operated once as an inverter generating an inverse voltage so as to avoid an undesirable short-circuit that may occur before the mechanical type persistent current switch 1 is turned on, and, in the second step, the mechanical type persistent current switch 1 is turned on in response to the ON instruction signal applied from the control unit 10. Thus, a considerably long period of time is required until the operation mode is shifted to the persistent current mode. It is well known that a mechanical type switch as described above requires a period of time generally longer than several hundred msec until the switch completes its operation after it receives an 0N instruction signal.
Thus, when the shift of the operation mode of the superconducting energy storage apparatus to the persistent current mode is delayed after the occurrence of trouble in the electric power system 9, the super-conducting coil 3 may discharge its stored energy to the electric power system 9 through the AC-DC converter unit 6, with the result that electrical apparatuses in the electric power system 9 may be heavily damaged.